Isoleucine-valine auxotrophs induced by transposable elements are being used in an investigation of the organization of the isoleucine-valine biosynthetic genes in Escherichia coli and Samonella typhimurium. Growth requirement and crossfeeding experiemnts have been found to be both more sensitive and easier to perform than enzyme assays on crude cell extracts. We have found evidence for an internal promoter in the ilvEDA operon between ilvE and ilvD in both E. coli ad S. typhimurium. Some S. typhimurium ilvE: :Tn 10 mutants have more ilvD and ilvA enzyme activity than others. We will determine whether the differences are correlated with either the sight of insertion or the orientation of the inserted element. We will determine whether this difference is in the level of promoter expression from mutant to mutant is also found in E. coli ilvE: :Tn10, ilvE: :Tn5 and/or ilvE: Mu mutants. S. typhimurium Tn10 insertion mutants believed to be defective in the isoleucine-specific acetohydroxy acid synthase have been found. The enzymatic defect in these mutants will be tested by appropriate enzyme assays and the mutants will be mapped. By using growth requirement and crossfeeding data in conjunction with enzyme assays on a large number of transposable element-induced isoleucine-valine auxotrophs of E. coli in these two genera and the differences between them. This will provide a solid groundwork for further investigations of the regulation of isoleucine-valine biosynthesis in prokaryotes.